It has long been common practice to manufacture screws and other threaded fasteners by roll-forming, and in recent decades similar roll-forming processes have been used in the manufacture of helical-tooth gears from cylindrical rod workpieces. In these well-known rolling processes, the toothed articles are formed by the relative motion of sets of dies which themselves have helical teeth that not only displace the metal on the surface of the rods but also pull the rod through the dies during the forming process. Some processes use a pair of flat dies that are reciprocated relative to each other, while others use sets of large cylindrical dies that roll in the same direction, squeezing the workpiece rod between them to form mating teeth on the surface of the rod as the rod rotates between them like a planetary gear.
Such known rolling dies have a "start" portion with a plurality of die teeth having tooth profiles designed with (a) predetermined pressure angles and (b) respective working depths that progressively increase from a first tooth through a final tooth of the start portion. The profile of the final tooth of the start portion is provided with a pressure angle and working depth substantially equivalent to the final profile geometry desired for the teeth of the workpiece being manufactured. Immediately following the start portion of such rolling dies is a "dwell" portion which also includes a plurality of die teeth; and the teeth of the dwell portion, like the final die tooth of the start portion, are also designed to have substantially the same profile geometry desired for the teeth of the workpiece that is being rolled.
A major problem that has affected such prior art rolling systems relates to a undesirable seam or fissure that is formed at the crest of the rolled tooth because the metal of the workpiece is squeezed radially outward faster along the tooth faces of the die teeth than it is moved outwardly in the spaces between the die teeth. This creates two peaks of work material at the roots of each pair of die teeth (i.e., at the top land of the formed tooth); and in the final stages of the roll-forming operation, these peaks fold over to form a seam. The resulting seam creates a potential weak spot that can cause a failure of the formed tooth under certain types of heavy loading.
In an attempt to overcome this problem, some prior art rolling dies (see U.S. Pat. No. 3,626,733 issued to E. S. Zook et al. in 1971, and U.S. Pat. No. 5,182,937 issued to J. F. Dickson in 1993) vary the pressure angle of the tooth profiles of the teeth used in the start portion of the rolling dies. Namely, the entire profile of the whole depth of the initial tooth of the start portion is provided with a pressure angle that is substantially larger than the pressure angle of teeth desired for the workpiece being manufactured; and then, as the respective whole depth of each successive tooth of the start portion gradually increases, this same larger pressure angle is gradually replaced by the final pressure angle desired for the workpiece being manufactured. That is, this successive, tooth-by-tooth replacement of the larger pressure angle by the final pressure angle begins at the lower end of the profile dedendum of one of the shorter teeth of the start portion while the remainder of the profile of the same tooth has the larger pressure angle. Thereafter, as this change progresses, all of the dedendum of a successive die tooth is formed with the final pressure angle while all of the addendum retains the larger pressure angle, until--for the die teeth near the end of the start portion--only the upper addendum retains the larger pressure angle; and finally, the entire tooth profile has been replaced with the final pressure angle.
The use of two different pressure angles, as just explained above, was intended to direct the flow of the workpiece metal in a manner that reduced its propensity to fold into a seam at the crest of the formed tooth. The invention herein is also directed to the elimination of the undesirable folding of the flowed metal of the workpiece and, thereby, to the manufacture of better quality gears by the rolling die method.